Mechanical and nanoindentation study of steel fibre - matrix interface in OPC and silica fume mortars

Macroscopic mechanical properties and nanomechanical properties of the interfacial transition zone (ITZ) were studied. Nine mortars with w/b = 0.3 and 0.5, silica fume/b = 0 and 0,1 and 0, 0,3 and 1 vol-% steel fiber were investigated (compressive-, three-point bending strength and nanoindentation tests). 1) Due to the hardness difference between particular steel fibre, aggregate and matrix, it is quite difficult to prepare samples for nanoindentation test and several grinding and polishing procedures were tried and the best possible grinding and polishing procedures for samples with water/binder ratios 0.3 and 0.5, for samples without and with steel fiber were determined. Samples 030000, 030003, 031003, 050000 and 050003 were selected for nanoindentation tests. The aggregate-matrix interfacial zone, steel fiber-matrix interfacial zone and steel fiber-matrix- aggregate interfacial zone were studied. After the nanoindentation test, the samples were removed, coated in carbon to get both Secondary Electron (SE) image and Backscattetred Electron (BSE) image of the indent areas. The whole indent size of each indent area in the SE image was determined; combined with the corresponding BSE image of the same indent area, the bond conditions cross the interfaces, the voids and microcracks within the indent area were observed. Then, the elastic modulus and hardness profiles of the interfacial transition zone were obtained. The following concluding remarks can be made from this experimental work: 1) Failure modes of the mortar are influence by the addition of steel fiber. Good ductile property is shown by mortars with steel fiber. Dissipated energy was used to measure the toughness of the mortar. Mortars with low water/binder ratio show higher toughness. The post elastic energy is always improved by the addition of steel fiber. 2) For samples with different water/binder ratios, the characteristics of the profiles of the elastic modulus and nanoindentation hardness of the interfacial transition zones are quite different. For sample with low water/binder ratio, efficient bonds across interfaces are shown by the rise in the hardness profile as the steel fiber and aggregate are approached. For sample with large water/binder ratio, due to the discontinuous bleeding voids underneath the fiber, the interfical bonds are not very good. 3) The characteristics of the profiles of the elastic modulus and nanoindentation hardness of the interfacial transition zones are also influenced by the addition of silica fume. In sample with 10% silica fume, in some parts of the sample, the addition of silica fume leads to no obvious presence of weak ITZ and the bond across the interfaces are efficient; while in other parts of the sample, such as the steel fiber-matrix-aggregate interfacial zone, the effect of the silica fume is not obvious. Thus, air content should be carefully controlled to decrease voids in interfacial transition zones of the sample. 4) The three-point bending strength and the toughness of the steel fiber reinforced mortar are related to the interfacial characteristics and microstructural morphology near the fiber-matrix interface. For mortar with the same w/b=0.3, comparatively lower bending strength and Welastic in mortar with 10% silica fume result from the wider thickness of the ITZ in the steel fiber-matrix-aggregate zone and voids and microcracks in this zone. Those discontinuous pores and the increase in microcracks may result from the reaction with silica fume and autogenous shrinkage. For mortar with different water/binder ratio, due to the poor bonds across the steel fiber-matrix interface and fiber-matrix-aggregate interface in mortar with w/b=0.5, the bending strength and fracture toughness are obviously lower than those of mortar with w/b=0.3.